1. Field of the Invention
The present invention generally relates to sheet transferring devices and image forming devices, and more specifically, to a sheet transferring device whereby plural sheets are transferred at a constant pitch in sequence, the sheet transferring device being used for a printer, copier, facsimile, or the like, and an image forming device having the sheet transferring device.
2. Description of the Related Art
A printer, copier, facsimile, or the like is used as an image forming device wherein a sheet is transferred to an image forming position and an image is formed on the sheet. In such an image forming device, if an image forming operation of an imaging system and operation of a sheet transferring device sending the sheet to the image forming position are not coordinated, an image offset is generated on the sheet. In continuous printing or continuous copying wherein plural sheets are continuously sent in turn so that image forming is made for each sheet, if a gap between a sheet and the following sheet is not constant, an image offset is generated.
If a transferring speed is different from a designed speed due to adhesion of dust, frictional wear of a transferring roller, or the like, the image offset may be generated. In addition, even if the transferring roller is new, when printing is performed continuously, a member of the transferring roller may expand or contract due to temperature change in the device, so that the designed speed may not be maintained.
Japanese Patent Application Publication No. 5-338845 discloses that difference between assumed time and measured transferring time is corrected by changing the transferring speed when the next sheet is transferred. More specifically, a feeding speed is adjusted so that a measured feeding time at a certain distance when the sheet is fed from a resist roller to a transferring position is a standard time. In other words, the feeding speed is adjusted to the standard speed.
Japanese Patent Application Publication No. 7-261485 discloses a sheet transferring device having a sheet sensor provided between a resist roller and a transferring position. A time when the sheet sensor detects the sheet sent from the resist roller at a low speed is measured. When the sensor detects the sheet, rotation of the resist roller is stopped. The difference between a standard time and a measured time is calculated. The resist roller is re-driven at the timing corresponding to the difference so that the sheet is adjusted to an image position on a photosensitive body and fed.
Japanese Patent Application Publication No. 11-165906 discloses a sheet transferring device having the following structure. In order to make a gap between sheets continuously fed securely constant, a time from when a sheet starts being fed from a resist roller to a transferring position to a time when a head end of the following sheet is detected by a detecting sensor is measured. If the measured time is shorter than the standard time, a paper feeding roller is stopped based on the difference.
Japanese Patent Application Publication No. 2001-206583 discloses an image forming apparatus having a determination sensor provided upstream of the resist roller so that a fed sheet reaches the resist roller at a standard timing. A sheet transferring speed upstream of the resist roller is adjusted corresponding to an offset amount against designated timing, which timing is when the determination sensor detects the sheet.
In a case where the sheet are fed from a paper feeding cassette, the head end of every sheet is scattered for every sheet. Hence, even if the sheets are transferred at an assumed speed, a transferring gap when the sheets are continuously transferred is not constant. In this case, even if transferring of the following sheet is adjusted by using the measuring result, since the transferring time is varied for every sheet, the transferring gap as a result of adjustment is not uniform.